This invention relates generally to ultrasound imaging systems that use ultrasonic transducers to provide diagnostic information concerning the interior of the body, and more particularly, to an apparatus and method for selectively optimizing an acoustic transducer.
Ultrasonic diagnostic imaging systems are in widespread use for performing ultrasonic imaging and measurements. For example, cardiologists, radiologists, and obstetricians use ultrasonic imaging systems to examine the heart, various abdominal organs, or a developing fetus, respectively. Diagnostic images are obtained from these systems by placing a scanhead against the skin of a patient, and actuating an ultrasonic transducer located within the scanhead to transmit ultrasonic energy through the skin and into the body of the patient. In response, ultrasonic echoes are reflected from the interior structure of the body, and the returning acoustic echoes are converted into electrical signals by the transducer in the scanhead.
FIG. 1 is a functional block diagram of an ultrasound imaging system 10 according to the prior art. The system 10 includes an ultrasound processor 11 that is coupled to a scanhead assembly 12 by a connecting cable 26. The ultrasonic processor 11 further includes a transmitter 22 that generates signals at ultrasonic frequencies for emission by the scanhead assembly 12, and a receiver 16 to process signals received by the scanhead assembly 12. In order to isolate the transmitter 22 from the scanhead assembly 12 while the receiver 16 is in operation, a transmitter isolation unit 18 decouples the transmitter 22 from the cable 26. Correspondingly, when the transmitter 22 is in operation, a receiver protection unit 19 decouples the receiver 16 from the cable 26. A controller 14 interacts with the transmitter 22, the receiver 16, the transmitter isolation unit 18 and the receiver protection unit 19 to coordinate the operation of these components. The controller 14 similarly interacts with a display system 15 to coordinate the reception of signals received by the processor 11 so that a visual image may be generated.
The scanhead assembly 12 includes a transducer assembly 28 that is comprised of one or more piezoelectric elements 30 that are capable of emitting ultrasonic pulses when excited by signals generated by the transmitter 22, and converting the reflected portions of the pulses into electrical signals that may be processed by the receiver 16. The transducer assembly 28 is coupled to the processor 11 through a tuning network 20 that tunes the assembly 28 to optimize the characteristics of the scanhead and the processor 11. The tuning network 20 may be attached to assembly 28 to form the integral scanhead assembly 12, or alternatively, the network 20 may be interposed between the assembly 28 and the processor 11 at a position along the connecting cable 26, as also shown in FIG. 1. Still further, the tuning network 20 may be positioned within the processor 11 (not shown) or within a connecting element in the connecting cable 26 (also not shown).
FIG. 2 is a partial schematic diagram of the ultrasound imaging system 10 according to the prior art. The transducer assembly 28 is serially coupled to the processor 11 through the connecting cable 26 and a tuning inductor 36. For clarity of illustration, FIG. 2 shows only a single element 30 (as shown in FIG. 1) coupled to a single connecting cable 26 by a single inductor 36. It is understood, however, that the transducer assembly 28 generally includes more than a single element 30, each of which may be coupled to the processor 11 through a separate, dedicated tuning inductor 36 and cable 26.
In general, the inductor 36 does not have an inductance value that permits the element 30 to be operated at only a single resonant condition. Instead, the inductor 36 is selected to allow the element 30 to be operated over a range of frequencies that define an acceptable operating bandwidth for the element 30 in a prescribed imaging mode. One trade-off of this approach is that a broad bandwidth for the element 30 generally results in a reduced sensitivity of the element 30 to the reflected pulses at a particular individual frequency. While somewhat reduced sensitivity of the element 30 may be acceptable when the imaging system 10 is operated, for instance, in a gray scale mode, it may have disadvantages in certain other ultrasound operating modes. For example, the system 10 may be operated in a Doppler ultrasound mode to provide an image of blood flow in an interior portion of a patient. In this imaging mode, the return signal is scattered from minute corpuscular components in the blood flow, which produces return signals that are generally greatly reduced in magnitude as compared to return signals typically encountered in the gray scale imaging mode. Increasing the magnitude of the emitted signal to produce stronger return signals cannot, in general, mitigate this disadvantage, since the magnitude of ultrasound signals cannot exceed prescribed levels that may produce cavitation effects in the interior portions of the patient""s body, or produce damaging levels of tissue heating. Alternatively, dynamically changing the inductance of the inductor 36 is difficult since the inductor 36 is generally a fixed component that is positioned within a scanhead assembly, or in other portions of an ultrasound imaging system.
Accordingly, there exists a need in the art for an ultrasound system that permits optimization of a transducer assembly to achieve wide bandwidth operation for certain ultrasound operating modes and narrower bandwidth operation to be selected for other modes of operation that require higher transducer sensitivity, as well other characteristics for different modes.
The invention is directed towards an apparatus and method for selectively optimizing an ultrasound transducer assembly to provide enhanced performance in specific ultrasound modes of operation. In one aspect, a variable impedance network is positioned between an ultrasound processor and a transducer assembly and may be coupled to the transducer assembly to form either a series or a parallel connection with the transducer assembly. The variable impedance network may be controlled by the processor to selectively alter the characteristics of the network to optimize the transducer assembly for a selected operating mode. In another aspect, the variable impedance network includes a pair of serially coupled inductors and a switch that permits one of the inductors to be controllably bypassed. In still another aspect, the variable impedance network includes a tapped inductor and a switch that permits the inductor tap to be controllably selected. In still another aspect, the inductor includes a tapped inductor having more than a single tap, each tap being selected by a switch to alter the impedance of the network.